Rechargeable batteries, which are manufactured from laminates of solid polymer electrolytes interposed between sheet-like electrodes, display many advantages over conventional liquid electrolyte batteries. These advantages typically include: lower overall battery weight; higher power density; higher specific energy; and longer service life. In addition, such batteries are also more environmentally friendly since the danger of spilling toxic liquid into the environment is eliminated.
EC cells generally include the following components: positive electrodes; negative electrodes; and an insulating material capable of permitting ionic conductivity, such as a solid polymer electrolyte, sandwiched between the electrodes. The negative electrodes, which are commonly referred to as anodes, are usually made of light-weight metallic foils, such as alkali metals and alloys, typically lithium metal, lithium-aluminum alloys and the like. The positive electrodes, which are commonly referred to as cathodes, are usually formed of a composite mixture of: an active material such as a transitional metal oxide; an electrically conductive filler, usually carbon particles; an ionically conductive polymer electrolyte material; and a current collecting element, usually a thin sheet of aluminum. Composite cathode thin films are usually obtained by coating the composite mixture onto a current collector.
Since solid polymer electrolytes are less conductive than liquid polymer electrolytes, solid or dry EC cells must be prepared from very thin films (e.g. total thickness of approximately 50 to 250 microns) to compensate the lower conductivity with a high film contact surface, thereby providing electrochemical cells with high power density. Each component of the EC cells must therefore be produced into very thin films of about 5 to 125 microns each.
Pure solid lithium, or solid lithium having a small percentage of alloy metals, is so ductile that it can be easily cut and worked at room temperature. The production of the lithium metal thin film is usually made by an extrusion process wherein an ingot of lithium/lithium alloy is inserted into a cylinder and pressed or pushed by an extrusion stem through a die aperture of the desired shape and thickness. The lithium/lithium alloy flows through a flow die of progressively narrowing cross-sectional area, thereby gradually shaping the metal flow toward its final desired shape. The metal flow subsequently exits through a flat faced die having an aperture featuring the desired cross sectional profile. In the particular case of a lithium metal anode, the profile is a thin and substantially rectangular one. Because of the requirement that the cylindrical ingot which enters the flow die must exit the latter as a thin film of substantially rectangular shape, manufacturers have to date been limited to produce lithium metal films of a width which does not exceed the diameter of the ingot itself. The size of the anodes so produced are therefore limited to the diameter of commercially available ingots.
The extrusion process of a lithium/lithium alloy ingot as described above must also be performed under vacuum since lithium is highly reactive, and it therefore easily oxidizes when exposed to the atmosphere. This is especially the case when it is heated and under pressure. The process of pushing the ingot along the walls of the cylinder chamber under high pressure generates sufficient heat for the lithium to react with ambient nitrogen and form nitrides (i.e., 6Li+N2→2Li3N) so that the process must be performed under vacuum. However, when the ingot has been almost completely extruded and a new ingot must be placed inside the cylindrical chamber, the chamber is opened thereby allowing ambient air to enter the chamber and react with the hot lithium left along the chamber's walls. For that reason, the typical lithium extrusion process includes the step of thoroughly cleaning the walls of the cylindrical chamber prior to extruding a new ingot in order to remove all nitrides which remain thereon. Otherwise, traces of hard nitrides could block the die opening and cause a split in the extruded lithium/lithium alloy sheet, thereby rendering the sheet unusable for the production of EC cell components.
Furthermore, the length of the lithium/lithium alloy film that can be produced by the prior art extrusion process is limited by the amount of material contained in a single ingot. This is so due to the fact that when a new ingot is placed inside the chamber, the remaining portion of the previous ingot (2–5 mm) must be removed since it cannot flow perpendicular to the pressure. Thus, the conventional lithium extrusion process produces a finite length of extruded lithium/lithium alloy sheet per ingot.
Considering this background, it clearly appears that there is a need for a process and apparatus adapted to produce a thin sheet or film of lithium/lithium alloy that alleviates the limitations imposed by the size and length of commercially available lithium/lithium alloy ingots.